Searching for the lost continent of Zealandia

ALL WET? Geologist Hamish Campbell believes New Zealand is just the tip of a large, previously completely submerged continent, but others disagree.

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Geologist Hamish Campbell's theories could be sunk  but that's exactly what he's set out to prove.

When Captain Cook farewelled his sponsors at Plymouth on August 26, 1768, he carried with him a sealed packet of secret instructions  to find the fabled Terra Australis Incognita, the great southern continent thought to be hiding somewhere in the South Pacific.

History says he failed, finding only the long finger of New Zealand. But GNS geologist Hamish Campbell says he succeeded. "In fact, he found it, it was just out of sight."

According to Dr Campbell's controversial new theory, the continent was Zealandia, a vast tract of continental crust of which modern New Zealand is only the emergent tip. In his new popular history, In Search of Ancient New Zealand (Penguin, pb $49.95), co-written with Gerard Hutching, and published today, he argues that Zealandia, and therefore New Zealand, might once have been under water.

Had Cook arrived 23 million years earlier, the graceful square-rigged Endeavour would have sailed southeast from Australia unobstructed, pitching and rolling clean over the submerged ancestral rocks of New Zealand.

If true, the theory plays havoc with the accepted family tree of New Zealand's unique wildlife, much as Dr Campbell says the sudden awakening and collision of the continental plates about 23 million years ago twisted and deformed the flat Zealandian plains and forced them up and out of their watery bed.

It's a stunning Wellington day  the harbour is flat enough for an afternoon waterski. But that's not what captures Dr Campbell's interest as we wander the waterfront. "See that range over there," he says, pointing to the Orongorongos. "That's so unbelievably young."

The ridge, he explains, is the bow wave of continental plate collision.

And then there's the Ruahines  on the crest of that range there's pure limestone, which can be formed only underwater, that's a mere two million years old.

"That means that less than two million years ago, the Ruahines were completely under the sea. Astonishing."

These are just some examples of the evidence Dr Campbell says points to total immersion. According to his theory, after Zealandia split from Gondwanaland 83 million years ago, the separation process stretched and thinned the Zealandia landmass, reducing its buoyancy and causing it to sink.

Continents are effectively floating on the oceanic crust below, like ice on water.

The thinner the ice, the lower it sits in the water.

While the rock record shows beyond doubt that most of Zealandia was eventually submerged, the question, says Dr Campbell, is whether any land or small islands remained.

"We cannot categorically say that there has always been land here. The geological evidence at present is too weak, so we are logically forced to consider the possibility that the whole of Zealandia may have sunk."

Dr Campbell's work on the history of the Chatham Islands has given rise to his theory, developed over a decade in collaboration with retired Otago University geologist Chuck Landis.

When he visits the Chathams, it's not an isolated island outpost or the historic home of the lost Moriori that he sees, it's Zealandia  son of Gondwanaland, father of New Zealand.

"The Chathams are very illuminating, there's nowhere else where you can see Zealandia untarnished. What you are looking at is Zealandia above water and undeformed (by continental plate movement)."

When it first split off from Gondwanaland, the continent would have been a flat, rolling country, between a half and a third the size of Australia.

Coal deposits suggest there would have been forested plains from about 75 million years to 45 million years ago.

It would have been a sleepy land; the plate boundaries that regularly shake Wellington, and the movement and collision of which have since pushed up modern New Zealand, were inactive.

Dinosaurs would have ranged freely, alongside mammals and plants carried across from Gondwanaland  beech trees and the ancestors of modern flowering heaths, hollies, lilies, pepper trees, mistletoes and totara, Dr Campbell says.

Until the maximum point of immersion, at 23 million years, life would have evolved away from the Gondwanan stock, adapting to life on the new continent and creating a unique Zealandian zoo.

Dr Campbell's theory may provide an answer to one of the great mysteries of New Zealand history  what happened to the mammals so notably absent from our native fauna. Though their fossil record is scant, Dr Campbell argues they must have been around after the split from Gondwanaland, because dinosaurs and mammals coexisted.

"We really don't have an answer, but maybe this idea of possible total submersion may actually be part of the explanation. We do need a mechanism to get rid of mammals."

Though it might provide some answers, the idea that New Zealand was fully drowned just 23 million years ago raises countless questions.

"Where did it come from? We've always had this understanding that there has always been land here. The plants and animals  surely they derived from a Gondwanan stock, right back to 83 million. That's sort of been a mantra and this idea actually challenges that.

"What we're now saying is that it is possible that the whole of our known native flora and fauna is derived from ancestral stock that is less than 25 million years old."

For biologist George Gibbs that's a step too far. There's nothing new, he says, in the idea that New Zealand was mostly submerged after its break-off from Gondwanaland, but complete immersion is a different matter  "there's overwhelming biological evidence against that".

Take the tuatara, thought to be a rare remnant of the dinosaur age. If the languid lizard swam, drifted or hitchhiked to New Zealand on seaborne debris, it must have come from somewhere.

Yet neither it nor the rest of New Zealand's wildlife appear in fossil records in Antarctica or Australia.

And then there's the pollen record, which is continuous and gives a reasonable indication of the types of plants living in the period after the Gondwanaland split.

While you would expect a marked change in plant life following complete submersion, none is seen, Dr Gibbs says.

The Chatham Islands, for example, are known to have been completely submerged, and have a very different, and less diverse, set of wildlife compared with New Zealand.

Most convincing, from Dr Gibbs's view, is the host of freshwater creatures  insects and freshwater crays  with clear ancestral links to South America and Australia. To them, salt water is poison. "You have to trace them back on the land."

But Dr Campbell is unfazed. It's not unreasonable, given what's known about evolution rates, to suggest that New Zealand's unique menagerie of ground-grubbing birds and unusual plants evolved in 20 million years rather than 80 million.

Nor is it impossible that their ancestors all arrived from afar to colonise the newly emerged land. "Tuataras relate to a very ancient group of reptiles, in many ways they're half dinosaur half lizard. Lizards get around. There are lizards on every rock in the ocean.

"They're cold- blooded, they're rafters. They're like crocodiles and snakes, they can get around much more readily than most of us appreciate."

The fossil record, argues Dr Campbell, is incomplete and can therefore be misleading  just because there are no known fossil tuatara remains elsewhere, doesn't mean they didn't exist.

And even the most unlikely creatures can travel great distances, with the help of nature or chance events. In the 1860s, waxeyes crossed the Tasman to New Zealand.

"Tiny little birds, nothing but a ball of feathers, and they managed to fly, or be transported in the wind, almost 2000 kilometres."

When the Manawatu River was heavily flooded in 2004, great chunks of turf were found bobbing out at sea. Such clay-bound soil, impenetrable by sea water, could provide a vehicle for hitchhiking freshwater creatures.

Dr Campbell says he can't be sure New Zealand was once fully drowned.

A true scientist, he gives a percentage probability  he's 80 to 90 per cent confident.

The real test of the theory will come in March, when the science behind the idea is published in international journal Geological Magazine, and when the science community will formally respond.

It's a dangerous question to ask a scientist at close proximity, but why does it really matter either way?

"Because we're all desperate to know what the future holds for us. And by examining the dung heap that we live on, we are beginning to see that, hell, the planet's been through a zillion climate changes of the sort we're now experiencing.

"There's nothing new under the sun. The memory banks of our planet can easily be tapped, and that's what we're doing."